" Perhaps the only good argument in favor of the current prison system is that it keeps violent people away from normal society."
TOTALLY false. An extremely small percentage of inmates are "life without parole", and since there is both a continuous stream of prisoners entering the system, there is also a continuous stream of prisoners coming back into society. While warehoused, with NO treatment, they often become more prone to violence, they become more disconnected and unskilled to fit into society, and thus more dangerous to the society they **WILL** be release back into.
Yes, as an "individual" they are kept from society for a time....and then returned to it. As a "system", given the one-in/one-out on a cyclical basis, the prison system in ejecting more violent, more hardened, less socially acceptable humans at the same rate as they bring them in (more or less) -- thus, the current warehousing systems in fact does more harm to society overall than it does good.